1. Field of the Invention
The present invention relates to vibratory seismic energy sources used for seismic prospecting. More particularly, it relates to a system for maintaining the relative position of the ground coupling plate of a seismic vibrator in the horizontal plane.
2. Description of the Prior Art
In seismic prospecting, it has become common to use, as a seismic wave source, an electrohydraulically controlled vibratory source, more simply referred to as a vibrator.
Typically, a vibrator comprises a double ended piston rigidly affixed to a coaxial piston rod. The piston is located in reciprocating relationship in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a reciprocating motion to the piston relative to the reaction mass. The piston rod extending from the reaction mass is rigidly coupled to a ground coupling plate (hereinafter baseplate) which is maintained in intimate contact with the earth material. The inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth. The motion of the piston is coupled through the piston rod and baseplate to impart vibratory seismic energy in the earth.
Typically, a vibrator is vehicle mounted, and to prevent decoupling of the baseplate from the ground during operation, a portion of the vehicle's weight is applied to the baseplate. The weight of the vehicle is frequently applied to the baseplate through one or more spring members so that a static bias force is imposed on the baseplate while the dynamic forces of the baseplate are decoupled from the vehicle. A support structure which rests atop the spring members supports the weight of the vehicle.
These spring members, usually in the form of air bags, may have little resistance to lateral stress. It is therefore necessary to employ a system to assure that the baseplate is maintained in vertical alignment with the vehicle. This horizontal stabilization should not interfere with or detract from, to any appreciable extent, the desired vibratory motion of the baseplate.
It is known in the art to use for this purpose a plurality of radius rods (see U.S. Pat. No. 3,690,402, issued Sept. 12, 1972). The radius rods extend both transversely and longitudinally and are connected generally in the horizontal plane with one end pivotally attached to the baseplate and the other end pivotally attached to the support structure. As the vibrator baseplate moves vertically, the radius rods do induce some horizontal displacement between the baseplate and the support structure. This horizontal displacement is, to a certain extent, taken up by rubber bushings in the eye ends of the radius rods. A portion of this induced horizontal motion, as well as other horizontal motion generated by interaction of the ground and the baseplate, is transmitted by the radius rods to the support structure and to the transport vehicle.
Another stabilization system, employing a Watt's Linkage, is comprised of two rods and a rotating center link. An end of each rod may be connected to the baseplate. The other end of each rod is connected to the rotating center link, which is pivotally connected to the support structure. The center link rotates freely about this pivotal connection and the two rods rotate freely about connections to opposing ends of the center link. The ends of the rods connected to the baseplate are also rotationally free. One of these structures is normally affixed to each side of the vibrator baseplate.
The Watt's Linkage system induces significantly less horizontal displacement of the baseplate resulting from vertical baseplate motion, than does the radius rods. Both the Watt's Linkage system and radius rods, however, may transmit undesirable vibrations from the baseplate to the support structure and transport vehicle, which may cause damage to the vehicle.